It is known that low density polyethylene (LDPE) made by the high pressure polymerization of ethylene, using a free radical initiator, can be cast into films using fast line speeds. The films so-produced lack sufficient impact strength and tensile properties for many of the applications for which a polymer film is desired. These LDPE polymers are characterized by the presence of long chain-branching along the polymer chains.
Linear low density polyethylene (LLDPE), which is known to be a linear polymer prepared using a coordination catalyst, contains a higher olefin comonomer (C.sub.3 -C.sub.12) interpolymerized into the polymer chain, but because of the substantial absence of long chain-branching, the interpolymer exhibits appreciably different properties when molten, and when solid, as compared to LDPE and HDPE. HDPE (a linear high density ethylene homopolymer) is made in substantially the same manner as LLDPE, but without the comonomer; it is the presence of the comonomer in the copolymer which causes the density of LLDPE to be lower than the HDPE homopolymer. In the presently described invention, the density of the LLDPE may be in the same density range as LDPE, or even lower, if enough of a given comonomer is used in the copolymer, or may be in an intermediate range (i.e. about 0.94 to about 0.955 gm/cc) when a lesser amount of the given comonomer is used. These linear polymers are made using a coordination catalyst, such as the Ziegler-type or Phillips-type catalyst, not with a free radical initiator such as used in making LDPE. The LLDPE polymers may be made in substantial accordance with, e.g. U.S. Pat. Nos. 4,302,566 or 4,076,698, especially the latter, so long as the catalyst and/or process conditions used are those which will produce the low ratio of I.sub.10 /I.sub.2 required for the present invention, such as by using the catalyst described in U.S. Pat. No. 4,612,300.
Whereas it is known that LDPE can be cast into films using fast line speeds, it is also known that stronger films are obtained with LLDPE. Persons attempting to cast LLDPE, in order to obtain stronger films, have encountered problems when trying to cast the films at line speeds as fast as that used with LDPE. The problems with using fast line speeds when casting films of LLDPE have been attributed to the lack of resistance to draw resonance.
The expression "draw resonance" or "surging" is defined in, e.g., U.S. Pat. No. 4,339,507 as ". . . a limit cycle corresponding to a sustained periodic oscillation in the velocity and cross-sectional area of a drawing process when the boundary conditions are a fixed velocity at the exit of an extruder and a fixed velocity at the take-off position. It occurs when the draw ratio exceeds a critical value. Draw resonance or surging can be thought of as an instability in take-off phenomenon of a material coming from an extruder, more or less homogeneously. The instability manifests itself in the occurrence of periodic fluctuations in the extrudate dimensions such as film thickness when a critical value of the take-up speed is exceeded. Surging may be so extreme as to actually break a web or film that extrudes from a die and totally shut down an extrusion coating process." The expressions "draw-down", "draw ratio", "melt strength", and "neck-in" are also defined in the patent. The patent discloses that LLDPE suffers from deficiences in processing which include problems with draw resonance and with high neck-in. The invention claimed in the patent is an extrusion coaiing process which uses a blend of LDPE polymer and LLDPE polymer to obtain the desired extrusion properties.
In U.S. Pat. No. 4,486,377 the draw resonance problem, found in drawing films of LLDPE, is said to be reduced by using a cooling gas between the die and the nip roll.
In an article titled "Reducing Draw Resonance in LLDPE Film Resins" by Peter J. Lucchesi et al, published in PLASTICS ENGINEERING, May, 1985, it is reported that LLDPE experiences resonance even at very low draw rates, largely due to its lack of long-chain branching. The article also states that ". . . The only known way to reduce the draw resonance of LLDPE is to cool the extrudate gradually rather than expose it to the shock-cooling of a water bath or chill-roll". The article suggests the use of a "draw resonance eliminator (DRE)" which involves the use of a fluid medium (air) against the molten film between the die and the nip and chill roll.
Authors E. H. Roberts et al in an article titled "New Processes For The Reduction of Draw Resonance in Melt Embossing And Extrusion Coating" published in 1985 in LAMINATIONS & COATINGS CONFERENCE/TAPPI Proceedings, also reported on the "Draw Resonance Eliminator (DRE)". The same authors also reported on the same subject in an article published in ANTEC, 1985.
In AlChE Journal (Vol. 24, No. 3), May 1978, on page 418, there is an article by Jae Chun Hyun titled "Part 1. Newtonian Fluids" which offers theoretical considerations related to draw resonance in melt spinning of fibers and films.
U.K. Patent G.B. No. 2,124,139 (published 2-15-84) discloses that LLDPE, having a propensity for draw resonance at high draw speeds, is drawn into a film using a draw roller located not more than 6 inches from the die so as to have a short draw-gap.
There is a perceived need for other means of avoiding draw resonance problems when extruding LLDPE which avoids having to add other polymers to it and avoids having to apply mechanical changes in the extrusion equipment whereby the drawing process is altered. The present invention substantially fills such need, as described hereinafter. However, if one already has made modifications in a cast film or extrusion coating process or equipment used therefor, or wishes to use a blend of LLDPE with another polymer (such as LDPE and the like), then the present invention can still be used to good advantage in such processes or equipment, or with such blends, and obtain the benefits of the combination of techniques.